1. Field of the Invention
The present invention relates to a charge controlling semiconductor integrated circuit of a secondary battery, and in particular, an IC (semiconductor integrated circuit) for charge control including a function of discharging electric charge of a bypass capacitor when power is off.
2. Description of Related Art
A charging apparatus of a secondary battery uses an IC mounted with a charge control circuit for controlling charging current with a transistor for controlling electric current including a metal oxide semiconductor field effect transistor (MOSFET, insulated gate field effect transistor, hereinafter referred to as MOS transistor) provided between an input terminal where direct current voltage is provided from a direct current power source such as an AC adaptor and an output terminal connected to the secondary battery.
In an IC for charge control, since direct current voltage input to the IC for charging is the power source voltage of an internal current of the IC, a bypass capacitor is provided between the voltage input terminal and ground point in an outer section of the chip to prevent malfunction of the internal current caused by noise coming into the input voltage.
In a charging apparatus of the secondary battery shown in FIG. 3A including a bypass capacitor when the AC adaptor is removed, as shown in FIG. 3B, the charge remaining in the bypass capacitor is supplied from the voltage input terminal to the internal circuit of the charge control IC and malfunction of the internal circuit occurs. For example, when a lighting driving circuit of the LED (light emitting diode) is internally included, there is a possibility that an undesired situation occurs where it is clear from the outside that a malfunction of the internal circuit occurred, for example the LED is lighted even when the power is off.
Especially when the capacity of the bypass capacitor is large or the electrical current consumption of the IC is small, as shown in FIG. 4A, since the time T1 for the charge remaining in the bypass capacitor to discharge is long, there is a problem that the span in which there is a possibility that the internal circuit malfunctions is long.
Incidentally, as a technique to discharge electric charge of the bypass capacitor when the power is off, there is a technique disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H6-6142. However, in such technique, discharge of the bypass capacitor connected to the output terminal is performed in order to synchronize the falling edge time of the output voltage, and the purpose of the technique is different from that of the present invention.